sgammo is one of my go to's for ammo, especially carry/sd ammo. Very reasonable prices and quick shipping. Another good site is palmetto state armory. They often have the cheapest price on different ammo.
As far as the AR, get a Vickers sling and a light for it. Shoot that thousand rounds, then you'll have a better idea if you want to add an optic, different handguard, different sights, trigger, etc. I wouldn't go off and spend hundreds of dollars on stuff just because; I've probably lost a couple g's doing that over the years already.
I'll eventually drop ALG triggers in my AR's, but I don't shoot them enough to justify spending money on something I don't shoot instead of pistol ammo.